While this has been touched on in many threads, I think it deserves its own.
At will casting, both through cantrips and rituals, as or more generous then 4E. Hybrid prepared/spontaneous casting. And, if that was not enough, spells that can be spontaneously cast at different levels.
Discuss...
Depends if you're looking at Full Casters only or all who used magic. If looking at the former, it is indeed as/more generous than 4E. If looking at the latter, it's vastly more restrictive (based on current evidence, which is relying on October playtest to some extent).
The really big difference between 4E and 5E is Ritual Casting, for me. In 4E, Ritual Casting is a Feat, and some classes get it for free, and it lets you cast any Ritual which you know (assuming you have the appropriate associated skill and components needed). Any character who doesn't get it for free can buy it (though they will also need the appropriate skills to cast the Rituals, and to learn the Rituals themselves, and so on).
Whereas in 5E, as of what we know right now, Ritual Casting is limited solely to what might be called "Full Casters", i.e. classes who focus largely on magic and have a full casting progression - Bard, Cleric, Druid and Wizard in the October playtest (and we know Cleric and Wizard remain the same on this in the Basic rules).
Even classes which can cast spells which are available as Rituals, but aren't those classes noted above, can't use Ritual magic (as of the October playtest).
Of course I'm really hoping this is just a "No PHB yet" thing, and we'll see a Feat which allows you some form of Ritual Casting - probably along the lines of "Pick a class, you can cast Rituals they can cast, but you have to learn each one and cast it from a Ritual Book which is like a Wizard's Spellbook" (I imagine that there would need to be other limits too - maybe make the "minimum level" to cast a Ritual 1 higher than the level of a full-caster PC to cast said Ritual).
ANYWAY, ignoring Ritual Magic, yes, it's pretty awesome. Spell slots feel very slightly more meta-game-y but somehow less bizarre than the "vanished from your memory" spells of yore. Cantrips are very well done this time out. I am disappointed that only Full Casters have them, but perhaps that's another Feat option? I wouldn't see 3-5 Cantrips from a specific class' list as being too much for a Feat, myself.
4E shows me that cantrips are unlikely to be abused - honestly, pretty much anything you can do with them by spamming them, you can do with a barrel of smokepowder or people hitting the object or setting stuff on fire or the like, so I'm not too concerned there. I do think several minutes of spamming might warrant a check of some kind, but I think it's unlikely to come up.
EDIT - If I just look at Wizards, this would definitely be my favourite version of them and their magic in any edition of D&D. Clerics I kind of preferred 4E, not out of strong feelings, but somehow the AEDU paradigm really worked with the idea of divinely-provided powers for me, whereas a spell slots seems a bit... prosaic. Not a big deal though.